The tram, due to open to passengers in 2019, will run from Circular Quay to Central, through Surry Hills and to Randwick and Kingsford.

From Friday night, George Street will be closed southbound between Grosvenor and Alfred streets, with one northbound lane staying open to allow access into The Rocks.

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George Street will be closed between Grosvenor and Alfred Street from Friday night due to construction of the light rail line.

Photo: Kate Geraghty

"This is really the zone that worries us," CBD Transport Coordinator General Marg Prendergast said. "It's just the potential that it will lock up this end of the city and lock up the buses. We don't know, we'll see next week."

"This is one of the trickier zones and we have the ability to gridlock the north," Ms Prendergast said. "It's the motorists blocking access for the buses, particularly in the afternoon, that we are worried about."

Chris Preston, Secretary of the Bus and Tram Division of the RTBU said the union had raised concerns about congestion on Bridge Street six months ago.

"The latest closure of George Street will just funnel more traffic into the area, making it harder for buses and all road users to get around," Mr Preston said.

"Bridge Street is already a carpark, with some buses waiting more than 15 minutes to turn onto the street."

Ms Prendergast said the stretch of George Street near The Rocks posed a unique challenge.

There are large numbers of visitors who are not familiar with local roads travelling to and from The Rocks, arrivals and departures at the Overseas Passenger Terminal can cause traffic to surge and there are more people driving to and from their work place.

"What you find down this end of town is it's got all the big businesses with all the carparks," Ms Prendergast said.

The northern part of the city is the biggest employment generator in the entire CBD and this is where people have all got their cars."

Ms Prendergast said commuters should not drive, and those who did should use alternate routes to leave The Rocks, including Hickson Road and Kent Street, during the afternoon peak

Transport for NSW is also working with big businesses in the area to encourage employees to travel outside of peak hours and to work from home or other locations.

From September, all of George Street will be shut from Circular Quay to Chinatown. The section of road near The Rocks will re-open in December, but drivers will only be able to use one lane in each direction.